The fracturing of earth formations in the vicinity of a well bore traversing earth formations is a commonly employed technique in the petroleum industry for stimulating the flow of oil or gas from the fractured formations. In the typical fracturing operation, large volumes of fluid, often water, are pumped downhole at high pressure to stress the formations of interest, usually through perforated casings. A solid particulate material, such as sand, is usually included as a proppant with the fluid to prop open the induced fractures in formations so that the formation fractures do not close when the pumping or stress pressure is released.
A radioactive tracer material can be used to label or identify either the fluid or the proppant which has been injected into the fractured formations. Thereafter, a gamma ray well log can be run to measure and record the resulting gamma ray activity as a function of depth for locating the radioactivity of the tracer and hence the location of the fracturing fluid or proppant. In complex fracturing operations where multiple zones are fractured or where "frac" fluids are injected in several stages, it may be desirable to inject and monitor multiple tracers in the fracturing operations. Multiple tracer uses might include, for example, injecting a different radioactive isotope into each zone, or at each stage of the operation, or placing radioactive isotopes onto the various solid and fluid components of the fracturing material. Monitoring of each of these radioactive tracer isotopes is desirable and can be used for an accurate analysis of the effectiveness of the fracturing operation, particularly as to determining the location, extent and radial location of the fractures.
Knowledge of the extent of a fracture in a formation as evidenced by information of the fracture configuration and the extent of the fracture in the direction radially away from the well bore is important in determining the success of a fracture operation. Such information can be used for optimizing future fracturing operations in other wells in the vicinity. Such information can also be of assistance in the diagnosis of post-stimulation problems.
In this respect, identification of a vertical placement of a tracer to indicate fracture travel in a vertical direction may be accomplished in some instances by gamma ray logging tools which are sensitive only to the overall presence of gamma rays. In some cases, however, the gamma ray log is inadequate because of its inability to distinguish between multiple tracer materials in place in the fracture and tracer materials inside the well bore or in channels or voids in the cement. Gamma ray spectroscopy can be used to discriminate in these cases, and to improve the estimates of vertical fracture travel. It has been used, as disclosed in U.S. Pat. No. 4,032,780, to detect gamma radiation from water flow behind a casing in a method for determining the distance from a detector in a well bore to the mean center of a water flow path behind casing. However, an indication of the extent of the fracture travel in a radial direction by a determination of a mean tracer penetration, i.e. the mean horizontal distance from the tracer in the formation to the detector in the logging tool has not heretofore been obtainable.
Gamma ray logs are often used in well logging for such purposes as detecting naturally occurring radioactive isotopes in downhole minerals, and hence the delineation of nonradioactive relative to highly radioactive minerals, or in connection with radioactive tracer operations. It is characteristic of gamma rays, that in passing through matter, the gamma rays experience successive Compton scattering interactions with the atoms of the material and lose energy with each interaction. After losing enough energy, they may be completely removed by interacting with atoms of the material through the phenomenon of photoelectric absorption. Natural gamma ray spectroscopy tools now in use in well logging operations can also measure the energy spectra of gamma rays emitted by radioactive tracers as they are detected by a gamma ray detector in the well. A radiation energy measuring tool of this type is described in a paper by Smith, H. D. Jr., Robbins, C. A., Arnold, D. M. Gadeken, L. L. and Deaton, J. G., "A Multi-Function Compensated Spectral Natural Gamma Ray Logging System," SPE Paper #12050, Fifty-Eighth Annual Technical Conference, San Francisco, Calif., Oct. 5-8, 1983. Each radioactive tracer material displays its own individual gamma ray spectrum or signature as affected by Compton scattering and photoelectric absorption phenomena. It is possible to accurately monitor multiple radioactive tracers by deconvolving the gamma ray spectral data into contributions from each individual radioactive tracer isotope as a function of depth. Such a technique is described in a paper by Gadeken, L. L. and Smith, H. D. Jr., entitled "TracerScan-A Spectroscopy Technique for Determining the Distribution of Multiple Radioactive Tracers in Downhole Operations," Paper ZZ, SPWLA Twenty-Seventh Annual Logging Symposium, Houston, Tex., June 9-13, 1986.